In recent years, as well as the market of mobile communication equipment such as a portable telephone growing, the service has become increasingly sophisticated. Along with this, the frequency band utilized by the communication network has shifted to a high frequency band of 1 GHz or higher, and also, there is a trend toward a multiple number of channels.
FIG. 1 is a circuit diagram illustrating a configuration of a related high frequency variable filter. The high frequency variable filter illustrated in FIG. 1 includes a plurality of channel filters 101a to 101c, and switches 102a and 102b. The passbands of the channel filters 101a to 101c differ from one another. A high frequency signal input from an input terminal 103 is output from an output terminal 104 via one channel filter selected by the switches 102a and 102b. By switching the switches 102a and 102b, it is possible to change the passband of the high frequency variable filter.
For example, Japanese Unexamined Patent Publications JP-A 10-335903 and JP-A 2007-174438 disclose the heretofore described kind of high frequency variable filter including the plurality of channel filters and the switches.
However, the configuration illustrated in FIG. 1 includes a number of filters equivalent to the number of channels. For this reason, as well as the size of the high frequency variable filter increasing, a cost also increases. Also, a loss of signal occurs in each switch.
In recent years, attention has been drawn to a small variable filter using an MEMS (Micro Electro Mechanical Systems) switch and an variable capacitor. An MEMS device such as an MEMS switch may be applied to a high frequency band variable filter with a high Q (quality factor).
“D. Peroulis et al, “Tunable Lumped Components with Applications to Reconfigurable MEMS Filters”, 2001 IEEE MTT-S Digest, p341-344”, “E. Fourn et al, “MEMS Switchable Interdigital Coplanar Filter”, IEEE Trans. Microwave Theory Tech., vol. 51, NO. 1, p320-324, January 2003”, and “A. A. Tamijani et al, “Miniature and Tunable Filters Using MEMS Capacitors”, IEEE Trans. Microwave Theory Tech., vol. 51, NO. 7, p1878-1885, July 2003″ disclose the heretofore described kind of MEMS device.
The MEMS device, because of its small size and low loss, is often used in a CPW distributed constant resonator (CPW: Coplanar Waveguide).
“A. A. Tamijani et al, “Miniature and Tunable Filters Using MEMS Capacitors”, IEEE Trans. Microwave Theory Tech., vol. 51, NO. 7, p 1878-1885, July 2003” discloses a filter with a structure in which a plurality of MEMS variable capacitors straddle three distributed constant lines. In this filter, by the variable capacitors being displaced to change a gap between the variable capacitors and distributed constant lines, it is possible to change the capacitance. By changing the capacitance of the capacitors, it is possible to change the passband of the filter.
In “A. A. Tamijani et al, “Miniature and Tunable Filters Using MEMS Capacitors”, IEEE Trans. Microwave Theory Tech., vol. 51, NO. 7, p1878-1885, July 2003”, quartz and glass are used as substrate materials. Also, the drive electrodes of the variable capacitors are disposed in a gap between a ground line and signal line formed on a substrate. Also, the length of the lines is defined by the permittivity of the substrate.
In the heretofore known distributed constant filter, the lower the frequency band, the larger the size. For example, the usable frequency band of principal mobile communication equipment such as a portable telephone is approximately 800 MHz to 6 GHz. However, when the frequency band is 800 MHz to 6 GHz, as the wavelength is long, the size of the distributed constant filter is too large for practical use. For example, in the event that a transmission line with an electrical length of λ/2 is fabricated to be a 75Ω microstrip line working at 800 MHz by using a ceramic substrate (permittivity ∈=9.4), the physical length being approximately 77 mm, it is difficult to put the filter into compact handheld wireless communication usage.
By using a high dielectric substrate, it is possible to shorten the length of the lines to some extent. However, when the substrate permittivity becomes higher, it not being possible to form a distributed constant line with a high characteristic impedance, there will be no degree of freedom in a filter configuration. For example, in the event that a microstrip line is formed using a substrate whose permittivity ∈ is 80, even though a distance between the signal line and ground is increased to 600 μm, a 50Ω (or other similar resistance) signal line may only take up a width of 20 μm. For this reason, a transmission loss increases. Consequently, there is a limit to reducing the filter size by increasing the substrate permittivity.